Large quantities of petroleum derived hydrocarbons are converted into higher value hydrocarbon fractions used as motor fuel by a refining process referred to as hydrocracking. In this process the heavy feed is contacted with a fixed bed of a solid catalyst in the presence of hydrogen at conditions of high temperature and pressure. This results in a substantial portion of the molecules of the feed stream being broken down into molecules of smaller size and greater volatility. The high economic value of petroleum fuels derived from these smaller molecules has led to extensive development of both hydrocracking catalysts and hydrocracking process technology.
The raw petroleum fractions typically charged to a hydrocracking process contain significant amounts of organic sulfur and nitrogen. The sulfur and nitrogen must be removed to meet modern fuel specifications. Removal or reduction of the sulfur and nitrogen is also beneficial to the operation of a hydrocracking reactor. The sulfur and nitrogen is removed by a process referred to as hydrotreating. Due to the similarity of the process conditions employed in hydrotreating and hydrocracking the two processes are often integrated into a single overall process unit having separate sequential reactors dedicated to the two reactions and a common product recovery section.